Conductive compositions are used for a variety of purposes in the fabrication and assembly of semiconductor packages and microelectronic devices. For example, conductive adhesives are used to bond integrated circuit chips to substrates (die attach adhesives) or circuit assemblies to printed wire boards (surface mount conductive adhesives).
The leading technique used throughout the electronics industry for soldering components to the substrate uses a metallic solder alloy which is so called eutectic solder containing by weight 63% tin and 37% lead. It is applied to the circuit board as a paste or a solder preform which is heated to above its melting temperature (183° C.) to let solder paste melt and form joint. Alternatively, the board is passed over a molten wave of solder to form joints to bond the electrical components to the circuit board. In either case, a flux material is used to remove surface oxidation from metallic surfaces and allow the molten solder to strongly bond to the surfaces and form reliable solder joints with excellent impact resistance. While the solder technology has existed for many decades, it has several shortcomings. Lead in the alloy is not environmental friendly. Numerous environmental regulations have been proposed to tax, limit, or ban the use of lead in the electronic solders. Secondly, high process temperature requires using more expensive thermo-stable substrate and does not fit flexible substrate that becomes more popular in the electronic industry. Another shortcoming is extra step to clean up the residue from flux material after reflow process which is an expensive and inefficient process.
Conductive adhesives offer several advantages over traditional solder assembly due to the absence of lead, low processing temperatures and a simplified assembly process that does not require solder flux and subsequent flux cleaning steps. Among the desired properties of conductive adhesives are long work life at room temperature, relatively low curing temperature and relatively short curing time, good rheology for screen printing, sufficient conductivity to carry an electric current when cured, acceptable adhesion to the substrate when cured, stable electrical resistance at high temperature and humidity over long periods of time, and good impact strength. Conductive adhesives are particularly useful for large area assemblies intended for various microwave applications which require bonding of a FR4 or ceramic circuit board to a metal substrate having an aluminum or copper core. In such applications, conductive adhesives provide electrical integrity, bond-line consistency and improved thermal dissipation that insures compliance with regulatory requirements, minimizes losses, minimizes distortion of high frequency digital signals and maintains low impedance to the ground plane.
Two types of conductive adhesives, film and paste, are commonly utilized. Film adhesives are utilized as low temperature process alternatives to solder and create a consistent ground path between an electrical circuit board and a metal backer. Film conductive adhesives are preferred for large area assembly due to their consistency in bond line thickness, low flow properties and availability as an intricate die cut part. Film adhesives are received by the end assembler in a B-stage condition having been pre-cut to the dimensions of the desired PCB. The carrier substrate, usually MYLAR, is removed from the film, the film is placed between the substrates to be bonded, and the package is bonded together under elevated temperature and pressure. Further, because film adhesives have a much higher viscosity than paste adhesives, settling of the filler is not a concern when the film is stored at room temperature for an extended period of time.
One issue surrounding film adhesives is their flexibility. Film adhesive having high strength are generally high modulus adhesives with limited flexibility. Such films having a high modulus and high cross-link density create a high stress condition due to extreme differences in the coefficient of thermal expansion between the board and the metal heat sink after cure. The result of the high stress is a high degree of warpage in the final assembly. Film adhesives that offer low stress are typically lower in modulus and also lower in adhesion strength. Consequently, commercially available flexible films are lower in adhesion strength than non-flexible films.
Paste conductive adhesives may also be used to bond together large areas, especially if the adhesive is screen printed onto the substrate. The screen printing enables high volume manufactures and limits waste. Paste conductive adhesives may also be B-stageable so that they may be pre-applied on the component in a manner similar to a film, stored, and then heated for final bonding. The rheology of paste adhesives must be monitored to insure accurate deposits of adhesive on the circuit boards. Likewise, the rheology of the paste must be monitored to avoid bleed out or slumping during the B-stage or cure processes.
It would be an advantage, therefore, to provide conductive film and paste adhesives that provide low stress and high adhesion to form electrically stable assemblies for use in semiconductor packaging operations. Advantageously, the adhesive would have a combination of superior adhesion to common flexible film adhesives and superior stress reduction as compared to common high strength film adhesives.